Field of the Invention and Related Art Statement
The present invention relates to an optical pick-up for writing and/or reading information on and/or from an optical record medium such as optical card and optical disk, and more particularly to a focusing error detecting device for use in the optical pick-up for writing and/or reading information on and/or from an optical card.
In Japanese Patent Application Laid-open Publications Kokai Sho Nos. 62-279,523, 63-91,830 and 63-117,334, there is disclosed a focussing error detecting device. In this known focusing detecting device, a light beam is projected onto an optical card from an inclined direction and a light beam reflected by the optical card is received by a photodetector including two light receiving regions which are divided in a direction in which a light spot on the photodetector moves in accordance with the focus condition of the light beam on the optical card. Then a focusing error signal is obtained by deriving a difference between output signals generated by these light receiving regions
The inventors of the instant application have experimentally found that the above mentioned known focusing error detecting device has the following drawback.
That is to say, in the known focusing error detecting device, the two light receiving regions receive light rays which are reflected by the optical card at portions which are situated at diametrically opposed peripheral areas of the light spot projected onto the optical card, so that the luminance of the light rays received by the light receiving regions is low. Therefore, the sensitivity of detecting the focusing error is low and the focusing error detection is liable to be affected by noise, so that a focusing servo could not be performed stably.
In Japanese Patent Application Laid-open Publication Kokai Sho No. 57-205,833 there is described another known optical pick-up including the focusing error detecting device. In this known optical pick-up, a light beam emitted by a laser light source is first made incident upon a refractive gratings to obtain three light beams. Then, these light beams are projected onto an optical record medium such as an optical card, a compact disk and a video disk by means of a polarizing beam splitter, a collimator lens, a .lambda./4 plate and an objective lens, and light beams reflected by the optical record medium are received by a photodetector by means of the objective lens, .lambda./4 plate, polarizing beam splitter, and further by means of concave lens and cylindrical lens. The photodetector is divided into four light receiving regions, and the focusing error signal as well as the tracking error signal are derived by processing output signals generated by the light receiving regions.
In this known optical pick-up, the concave lens is provided for separating the three light beams reflected by the optical record medium and the cylindrical lens is arranged for introducing into the reflected light beam an aberration which is required for detecting the focusing error. Therefore, the construction of the optical pick-up is liable to be large and complicated, and the cost is increased.
Further in the known optical pick-up, the light beam emitted by the laser light source is converted into a parallel light beam, by means of the collimator lens and then the parallel light beam is made incident upon the objective lens. The optical record medium is placed on the focal point of the objective lens, so that the light spot having a very small diameter is projected onto the optical record medium. In an optical card disclosed in the above mentioned Japanese Patent Application Laid-open Publication Kokai Sho No. 63-7,533 there is disclosed an optical card in which a number of track units are formed in parallel with each other and in each of the track units there are formed a plurality of tracks. In such an optical card, a plurality of tracks in a track unit has to be read out simultaneously. However, when the optical card is placed at the focal point of the objective lens, it is difficult to effect the data reading accurately.